This invention relates to steam generators equipped with air pollution control devices and more particularly to an apparatus and method for providing a flow path directly to the stack thereby bypassing the air pollution control equipment.
Air pollution control equipment is being installed on all coal-fired steam generators in order to remove from the flue gas particulate matter and gaseous pollutants such as SO.sub.2 which are inherently formed during the combustion process. Frequently, the air pollution control equipment installed includes a gas scrubber disposed downstream of the induced draft fan, said scrubber designed to remove SO.sub.2, and often particulate matter also entrained in the flue gas. In operation, the combustion products formed in the furnace, termed flue gas, exit the boiler through an air preheater to an induced draft fan which raises the pressure of the flue gas to a level sufficiently above atmospheric pressure to ensure proper venting of the flue gas through the stack. Upon leaving the induced draft fan, the flue gas, before continuing to the stack, passes through the scrubber wherein the SO.sub.2 and particulate matter are removed.
Many coal-fired steam generator furnaces are also designed to fire clean fuel such as natural gas or low sulfur oil which do not produce levels of particulate matter or sulfur oxides high enough to necessitate tail end flue gas cleaning. When these clean fuels are being fired, it is desirable to operate the steam generator without the scrubber in service and provide a flow path, commonly termed a scrubber bypass, for venting the flue gas around the scrubber directly to the stack. However, because of the strict governmental air pollution regulations limiting the emissions of SO.sub.2 and particulate matter, the scrubber bypass must be closed off when coal is being fired to ensure that contaminated flue gas does not leak through to the atmosphere when a scrubber is in operation.
A common means for controlling the flow of flue gas through the scrubber bypass is a multi-bladed louver-type scrubber bypass damper placed in the bypass to the stack. This scrubber bypass damper when closed blocks the flow through the bypass thereby forcing the flue gas to flow through the scrubber, and when opened allows the flue gas to bypass the scrubber and flow directly to the stack. An additional multi-bladed louver damper is placed in the inlet to the scrubber and operates in coordination with the scrubber bypass damper, opening when the scrubber bypass damper is closed, i.e., when the scrubber is in operation and closing when the scrubber bypass damper is opened, i.e., when the scrubber is out-of-service.
A major problem associated with this prior art arrangement is the over-pressurization of the furnace which can result if the scrubber bypass damper fails to open when the scrubber inlet damper is closed as the scrubber comes off line. In such a case, both of the flow paths to the stack will be blocked to flue gas flow by their respective closed dampers; the furnace pressure would rise to an unacceptable level causing shutdown and potential damage to the boiler or its support structure. Recognizing this problem, elaborate control systems have been developed and installed in an attempt to provide highly reliable, fail-safe operation of such multi-bladed louver dampers.